1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell having a cathode, an anode, a non-aqueous electrolyte and a separator. More particularly, it relates to a non-aqueous electrolyte secondary cell in which a separator is of a multi-layer structure having two or more layers.
2. Description of Related Art
Recently, variegated portable electronic equipment, such as camera built-in type VTR (video tape recorder), portable telephone sets or lap top computers, have made their debut and, with remarkable progress in the electronic technology, small-sized lightweight versions of these electronic equipment have been realized one after another. As a portable power supply for these electronic equipment, researches and developments for improving the energy density of the cells, in particular the secondary cells, as portable power sources for these electronic equipment, are going on briskly.
Of these, lithium ion secondary cell, for example, is thought to be promising because this cell is able to develop a higher energy density than that achievable with a nickel cadmium cell as a conventional aqueous electrolyte secondary cell. As the separator for the non-aqueous electrolyte secondary cell, such as lithium secondary cell, a polyolefin micro-porous film, as typified by high molecular polypropylene or high molecular polypropylene, is widely used.
It should be noted that, when the temperature of the non-aqueous electrolyte cell becomes higher than a preset temperature to reach a shutdown temperature, the polyolefin micro-porous film, used as a separator for the non-aqueous electrolyte cell, is fused to stop the numerous pores to obstruct lithium ion migration to forcibly halt the cell reaction, as a result of which the inner temperature of the cell is prevented from rising to maintain the safety of the non-aqueous electrolyte cell.
There are micro-irregularities of several μm in the surface of a plate of the lithium cell. Thus, if a polyolefin based porous film, liable to be torn, is built into the lithium cell as a separator for the cell, there is a risk that the film is damaged by the micro-irregularities on the plate surface to cause electrical shorting.
Although it has been proposed to use two sorts of porous films having different values of mechanical strength to improve the film strength, this approach cannot be said to give sufficient results.
It should be noted that, as the polyolefin micro-porous film, used in this non-aqueous electrolyte secondary cell, such a film having the average pore diameter on the order of 1 to 0.05 μm and a porosity on the order of 45%, is widely used, depending on the material used. With the non-aqueous electrolyte secondary cell, in which the separator has these micro-sized pores, lithium ions can be migrated across the cathode and the anode in charging/discharging the cell.
On the other hand, when the non-aqueous electrolyte secondary cell is overcharged such that the inner temperature of the cell is raised, this polyolefin micro-porous film exhibits a so-called shutdown effect in which the cell undergoes an endothermic reaction and is thereby melted, with the micro-sized pores being then closed to cause the current to cease to flow.
However, when the value of the porosity of the separator, comprised of a polyolefin micro-porous film, is high, there is a certain time lag until separator pores are completely blocked since the separation reaches the shutdown temperature. Thus, there is a possibility that inner temperature in the cell is further raised during this time interval, with the separator being melted and flowing out on reaching its meltdown temperature. In such case, there is a possibility of electrical shorting in the non-aqueous electrolyte secondary cell due to physical contact between the cathode and the anode.
So, the porosity of the separator is desirably set to a lower value. However, in this case, lithium ions are hardly movable across the cathode and the anode during charging/discharging of the non-aqueous electrolyte secondary cell, with the result that its low temperature characteristics are deteriorated.
Thus, there has as yet not been established a non-aqueous electrolyte secondary cell having optimum low temperature characteristics and high safety against over-charging.
On the other hand, if a micro-sized-pore polyolefin film having a low melting point is used as a separator, it is poor in physical strength against e.g., punching, piercing, thus raising the inferiority fraction in manufacturing the cells.